4.1.4 Long distance transport of water

I. Introduction

Definition of Long Distance Transport of Water

• Long distance transport of water refers to the movement of water and other solutes from the roots to the shoots of a plant through the xylem and phloem tissues.

Importance of Long Distance Transport of Water in Plants

• Water and Nutrient Distribution: Water is essential for plant growth and survival, and long distance transport of water is necessary for the distribution of water and nutrients throughout the plant.
• Turgor Pressure and Cell Growth: Long distance transport of water helps to maintain turgor pressure in plant cells, which is important for cell expansion and growth.
• Role in Photosynthesis: Water plays a crucial role in photosynthesis, as it is a reactant in the process.
• Regulation of Plant Temperature: Long distance transport of water is also important for the regulation of plant temperature, as water is involved in the process of transpiration, which helps to cool the plant.
• Overall Importance: Overall, long distance transport of water is essential for the proper functioning and survival of plants.

II. Mechanism of Water Absorption by Plants

Root Hairs

• In higher plants, water absorption occurs through the root hairs, which are extensions of the root epidermal cells.
• Root hairs increase the surface area of the roots, allowing for more efficient water absorption.

Factors Affecting Water Absorption

• Water absorption by plants is affected by various factors, including:
  • Soil water potential: Water moves from areas of high water potential to areas of low water potential, so plants absorb water from soil with a higher water potential.
  • Temperature: Higher temperatures can increase the rate of water absorption by plants.
  • Root surface area: Plants with larger root surface areas can absorb more water.
  • Soil texture: Soil texture affects the ability of water to move through the soil and be absorbed by roots.

Mechanism of Water Absorption

• Water absorption by plants occurs through two mechanisms: passive and active.
• Passive water absorption occurs due to the loss of water in transpiration, which creates a tension or low water potential in the xylem channels. This tension pulls water from the soil into the roots.
• Active water absorption requires energy from respiration and involves the movement of water against a concentration gradient.
• Water absorption from the soil and its inward movement may also occur due to osmosis.

III. Apoplast and Symplast Pathways in Water Transport

Definition of Apoplast and Symplast Pathways

• Apoplast pathway: This pathway involves the transport of water and nutrients through the non-living components of the plant, such as the cell walls and intercellular spaces.
• Symplast pathway: This pathway involves the transport of water and nutrients through the living components of the plant, such as the cytoplasm and plasmodesmata.

Differences between Apoplast and Symplast Pathways

• The apoplast pathway involves the transport of water and nutrients through the non-living components of the plant, while the symplast pathway involves the transport of water and nutrients through the living components of the plant.
• In the apoplast pathway, water and nutrients move through the cell walls and intercellular spaces, while in the symplast pathway, they move through the cytoplasm and plasmodesmata.
• The apoplast pathway is faster than the symplast pathway, as it does not require the movement of water and nutrients through living cells.

Importance of Apoplast and Symplast Pathways in Water Transport

• Both the apoplast and symplast pathways are important for the transport of water and nutrients in plants.
• The apoplast pathway is important for the transport of water and nutrients across tissues and organs, while the symplast pathway is important for the transport of water and nutrients within cells.
• The apoplast and symplast pathways work together to ensure efficient water and nutrient transport in plants.

IV. Transpiration Pull in Plants

Definition of Transpiration Pull

• Transpiration pull is the force that drives the movement of water through the xylem of a plant from the roots to the leaves.
• It is caused by the evaporation of water from the leaves, which creates a negative pressure or tension that pulls water up through the xylem.

Mechanism of Transpiration Pull

• Water is absorbed by the roots and transported through the xylem to the leaves, where it is lost through transpiration.
• As water is lost through transpiration, it creates a negative pressure or tension in the xylem, which pulls more water up from the roots.
• The cohesive and adhesive properties of water also contribute to the movement of water through the xylem.

Factors Affecting Transpiration Pull

• Environmental factors such as temperature, humidity, and wind speed can affect the rate of transpiration and therefore the strength of the transpiration pull.
• The size and number of leaves on a plant can also affect the rate of transpiration and the strength of the transpiration pull.
• The availability of water in the soil can also affect the rate of transpiration and the strength of the transpiration pull.

V. Root Pressure in Plants

Definition of Root Pressure

• Root pressure is the transverse osmotic pressure within the cells of a root system that causes sap to rise through a plant stem to the leaves.
• Root pressure occurs in the xylem of some vascular plants when the soil moisture level is high either at night or when transpiration is low during the daytime.

Mechanism of Root Pressure

• Root pressure is caused by the active distribution of mineral nutrient ions into the root xylem, which lowers the water potential and causes water to diffuse from the soil into the root xylem through osmosis.
• Root pressure provides a force that pushes water up the stem, which can transport water and dissolved mineral nutrients from roots through the xylem to the tops of plants.

Factors Affecting Root Pressure

• Root pressure is affected by various factors, including:
  • Soil moisture level: Root pressure occurs when the soil moisture level is high either at night or when transpiration is low during the daytime.
  • Active distribution of mineral nutrient ions: The active distribution of mineral nutrient ions into the root xylem is necessary for root pressure to occur.
  • Transpiration rate: When transpiration is high, xylem sap is usually under tension, rather than under pressure, due to transpirational pull.

VI. Guttation in Plants

Definition of Guttation

• Guttation is a natural process in plants where excess liquid, including water and dissolved materials, is pushed out or secreted from the leaves or blades.

Mechanism of Guttation

• Guttation occurs due to the buildup of root pressure, which forces xylem sap to excrete through openings on the leaf surface called hydathodes.
• The buildup of root pressure is caused by the active distribution of mineral nutrient ions into the root xylem, which lowers the water potential and causes water to diffuse from the soil into the root xylem through osmosis.
• Guttation typically occurs at night when transpiration is low and the soil moisture level is high.

Factors Affecting Guttation

• Environmental factors such as temperature, humidity, and wind speed can affect the rate of guttation in plants.
• The size and number of leaves on a plant can also affect the rate of guttation.
• The availability of water in the soil can also affect the rate of guttation.

VII. Xylem and Phloem Transport in Plants

Definition of Xylem and Phloem

• Xylem and phloem are two types of complex tissues that form the vascular system of plants.
• Xylem is responsible for transporting water and minerals from roots to various parts of the plant, while phloem is responsible for transporting sugars made by photosynthetic areas of plants to storage organs like roots, tubers, or bulbs.

Role of Xylem and Phloem in Long Distance Transport of Water

• Xylem and phloem work together as a unit to bring about effective transportation of water, nutrients, minerals, and food throughout the plant.
• Xylem transports and stores water and water-soluble nutrients in vascular plants, while phloem is responsible for transporting sugars, proteins, and other organic molecules in plants.
• Xylem and phloem form vascular bundles, which are found in the leaves, stems, and roots of plants.

Differences between Xylem and Phloem Transport

• Xylem and phloem have different structures and functions.
• Xylem is made up of dead cells, while phloem is made up of living cells.
• The movement of xylem is unidirectional, while the movement of phloem is bidirectional.
• Xylem transports water and minerals from roots to various parts of the plant, while phloem translocates sugars made by photosynthetic areas of plants to storage organs like roots, tubers, or bulbs.

VIII. Factors Affecting Long Distance Transport of Water in Plants

Environmental Factors Affecting Long Distance Transport of Water

• Environmental factors such as temperature, humidity, and wind speed can affect the rate of transpiration and therefore the long-distance transport of water in plants.
• Elevated levels of carbon dioxide can increase leaf temperature, leading to increased water transport through the transpiration stream in many plants.
• Light intensity can also affect the rate of transpiration and therefore the long-distance transport of water in plants.

Plant Factors Affecting Long Distance Transport of Water

• The size and number of leaves on a plant can affect the rate of transpiration and therefore the long-distance transport of water in plants.
• The availability of water in the soil can affect the rate of water absorption by the roots and therefore the long-distance transport of water in plants.
• The structure and function of xylem and phloem tissues can also affect the long-distance transport of water in plants.